How to choose between grit washing or grit classification

The primary benefit to grit removal in the wastewater treatment process is to eliminate potential damage to downstream mechanical equipment, and reduce the likelihood of adverse effects on the treatment processes.

There are many different types of grit removal systems currently utilized in the municipal wastewater industry, including mechanical vortex, induced vortex, multi-tray vortex, aerated grit chambers and detritus tanks. Regardless of the methodology used to collect the grit, the need still exists for dewatering.

When designing grit systems, there are two options available for dewatering prior to disposal, either grit “washing” or grit “classification”. In simplified terms, you can “wash” the grit to reduce organics (typically <5%), or you can simply “dewater” it and not address the organics (typically <25%). The decision is normally dictated by tolerance for odours directly related to the percentage of organics and moisture content of the discharged grit.

How a grit washer works

A vortex grit washer receives direct pumped flow into a tangentially fed vortex style tank from either a grit pump or airlift, without the need for primary separation. It can operate effectively over a wide range of flows, with standard flows up to 640 gpm.

Due to the grit washer operating principles, mechanical agitator and internal grit scour wash system, the organics are “washed” and rejected. The cleaned grit is transported up a 40 degree inclined screw conveyor, resulting in an extremely dry, clean and odour-free grit, with very low organics (volatile solids) content of <5%.

Key components of a typical grit washer.

How a grit classifier works

Grit classification is available in two operational styles: “dry” or “wet”.

A “dry” classifier includes a cyclone separator to concentrate the grit and discharge the underflow from the cyclone to further dewater as it is being discharged via an inclined screw conveyor. Typically, cyclone classifiers can have a higher percentage of organics in the grit discharge, somewhere in the range of 10% – 15%. The moisture content is in the range of 25% – 30%.

Limitations when considering a cyclone classifier include: a limited range of flow based on cyclone size and corresponding operating pressure, and their inability to operate with an airlift design, as they cannot maintain a constant pressure.

A “wet” classifier is fed a water/grit slurry directly from the grit basin. It includes a large flared settling zone to allow the grit to settle and dewater as it is being discharged via an inclined screw conveyor. Typical “wet” classifiers can retain an even higher percentage of organics in the grit discharge in the range of 20% – 25%, with a higher moisture content of 35% – 45%.

Conclusion

A number of factors should be considered before making a final selection of grit removal equipment, including: costs, flow rate, moisture content, and tolerance for odours due to organics in the discharged grit. The key design parameters are summarized in Table 1.

ES&E Magazine provides vital information for professionals that are engaged in the design, construction and operation of municipal water and wastewater treatment systems, stormwater management, industrial/hazardous waste management and air pollution.